Pre-driver for bridge circuit

BACKGROUND OF THE INVENTION

p-0002The present invention relates to a pre-driver, and more particularly, to a pre-driver for connecting a load such as a motor to a drive circuit.

p-0003A bridge driver is known in the art as a drive circuit for driving a motor. A half-bridge circuit, which is formed by two NMOS transistors, or a full-bridge circuit (also known as “H-bridge”), which is formed by four NMOS transistors, normally make up the bridge driver. In a half-bridge circuit, two transistors are connected in series between a high potential power supply and ground. Output voltage generated at an output node between the two transistors is supplied as an operational voltage to a load. In a full-bridge circuit, a first set of two series-connected transistors and a second set of two series-connected transistors are connected in parallel between a high potential power supply and ground. A first output voltage, which is generated at a node between the first set of transistors, and a second output voltage, which is generated at a node between the second set of transistors, are supplied to a load. A pre-driver is used as a gate drive circuit that drives the transistors of a bridge driver such as a half bridge circuit or a full bridge circuit.

p-0004Japanese Laid-Open Patent Publication No. 2005-354586 describes a pre-driver that drives a high-side transistor (MOS transistor connected to high power supply) arranged in a bridge driver. The pre-driver charges a gate capacitor formed between the source and gate of the high-side transistor with constant current. This prevents the gate drive voltage supplied to the high-side transistor from exceeding the source-gate withstand voltage (gate voltage) of the high-side voltage.

p-0005However, in the pre-driver described in the above publication, the rise speed of the gate drive voltage is restricted to be less than or equal to a constant speed to prevent the gate drive voltage from exceeding the gate voltage. This speed restriction effects the responsiveness of the circuit. The responsiveness may be improved by increasing the speed (current amount) used to charge the gate capacitor. However, increasing the current could damage the gate. Thus, to increase the charge current, the circuit scale of the pre-driver would have to be enlarged.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic circuit diagram showing a preferred embodiment of a pre-driver according to the present invention for driving a high-side transistor in a half-bridge circuit;

<figref idrefs="DRAWINGS">FIG. 2A</figref> is a waveform diagram showing the output voltage of the half-bridge circuit and internal power supply voltage of the pre-driver; and

<figref idrefs="DRAWINGS">FIG. 2B</figref> is a waveform diagram showing differences between the internal power supply voltage of the pre-driver and output voltage of the half-bridge circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

p-0010In the drawings, like numerals are used for like elements throughout.

p-0011The present invention provides a pre-driver for generating drive voltage for driving a high-side transistor of a bridge driver that does not exceed the device withstand voltage of the high-side transistor.

p-0012One aspect of the present invention is a pre-driver for connection to a bridge driver that generates a bridge driver output voltage at an output node between a first drive transistor connected to a first high-voltage power supply and a second drive transistor connected between the first drive transistor and ground. The pre-driver includes a reference circuit for generating reference voltage that is varied in a manner dependent on the output voltage of the bridge driver. A regulator circuit generates, in response to the reference voltage, internal power supply voltage that is substantially higher than the output voltage by a constant voltage. A buffer circuit generates drive voltage that drives the first drive transistor based on the internal power supply voltage and the output voltage.

p-0013A further aspect of the present invention is a pre-driver for connection to a bridge driver that generates a bridge driver output voltage at an output node between a first drive transistor connected to a first high-voltage power supply and a second drive transistor connected between the first drive transistor and ground. The pre-driver includes a reference circuit, connected between the output node and a second high-voltage power supply that supplies higher voltage than the first high-voltage power supply, for generating reference voltage that is varied in a manner dependent on the output voltage of the bridge driver. A push-pull circuit, connected between the output node and the second high-voltage power supply and including an output terminal and an input terminal that receives the reference voltage, generates internal power supply voltage that is substantially higher than the output voltage by a constant voltage in response to the reference voltage. A current adjustment circuit, connected between the second high-voltage power supply and the input terminal of the push-pull circuit, adjusts current that flows to the input terminal of the push-pull circuit. A buffer circuit generates drive voltage that drives the first drive transistor based on the internal power supply voltage and the output voltage.

p-0014Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

p-0015A preferred embodiment of a pre-driver <b>10</b> according to a preferred embodiment of the present invention will now be discussed with reference to <figref idrefs="DRAWINGS">FIGS. 1</figref>, <b>2</b>A and <b>2</b>B. <figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic circuit diagram of the pre-driver <b>10</b> connected to a half-bridge circuit <b>100</b>, which is one type of a bridge driver.

p-0016Referring to <figref idrefs="DRAWINGS">FIG. 1</figref>, the half-bridge circuit <b>100</b> includes a high-side transistor Q<b>1</b> (first drive transistor) and a low-side transistor Q<b>2</b> (second drive transistor), which are connected in series between a first high-voltage power supply <b>110</b> and ground. In the preferred embodiment, the transistors Q<b>1</b> and Q<b>2</b> are formed by N-channel MOS transistors. More particularly, the transistor Q<b>1</b> has a drain connected to the first high voltage power supply <b>110</b>, a source connected to an output terminal OP of the half-bridge circuit <b>100</b>, and a gate for receiving first drive voltage VDH. The transistor Q<b>2</b> has a source connected to ground, a drain connected to the output terminal OP, and a gate for receiving second drive voltage VDL. A load (e.g., motor) is connected to the output terminal OP. The transistors Q<b>1</b> and Q<b>2</b> are operated in a complementary manner by the drive voltages VDH and VDL. As a result, output voltage OUT having the level of the first high-voltage power supply <b>110</b> or the ground level is supplied to the load from an output node (the output terminal OP) between the two transistors Q<b>1</b> and Q<b>2</b>.

p-0017The pre-driver <b>10</b>, which is connected to the half-bridge circuit <b>100</b>, generates the first drive voltage VDH to drive the high-side transistor Q<b>1</b>. The second drive voltage VDL, which drives the low-side transistor Q<b>2</b>, is generated by another pre-driver (not shown). The pre-driver <b>10</b> may, of course, include a circuit that generates the second drive voltage VDL.

p-0018As shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, the pre-driver <b>10</b> is connected to a second high-voltage power supply <b>12</b>. The second high-voltage power supply <b>12</b> provides the pre-driver <b>10</b> with second power supply voltage VG, which is higher than first power supply voltage VM supplied to the half-bridge circuit <b>100</b> by the first high-voltage power supply <b>110</b>. The second power supply voltage VG is set to a value corresponding to, for example, “first power supply voltage VM+Vgst”. Here, Vgst represents the source-gate withstand voltage (hereafter referred to as the “gate withstand voltage”) of the high-side transistor Q<b>1</b> in the half-bridge circuit <b>100</b>. That is, the second power supply voltage VG is set based on the gate withstand voltage Vgst of the transistor Q<b>1</b> and the first power supply voltage VM so that the pre-driver <b>10</b> can drive the transistor Q<b>1</b> even if the output voltage OUT rises to the first power supply voltage VM. The second power supply voltage VG is generated by, for example, an external charge pump (not shown).

p-0019The pre-driver <b>10</b> includes a reference circuit <b>20</b>, a regulator circuit <b>30</b>, a level shifter <b>50</b>, and a buffer circuit <b>60</b>. Preferably, the pre-driver <b>10</b> further includes a current adjustment circuit <b>40</b>, as shown in <figref idrefs="DRAWINGS">FIG. 1</figref>. The reference circuit <b>20</b> is connected between the second high-voltage power supply <b>12</b> and the output terminal OP of the half-bridge circuit <b>100</b>. The reference circuit <b>20</b> includes at least one Zener diode. For example, the reference circuit <b>20</b> of the preferred embodiment includes two series-connected Zener diodes D<b>1</b> and D<b>2</b>.

p-0020The anode of the Zener diode D<b>2</b> is connected to the output terminal OP of the half-bridge circuit <b>100</b> via a resistor R<b>4</b>. The cathode of the Zener diode D<b>2</b> is connected to the anode of the Zener diode D<b>1</b>. The cathode of the Zener diode D<b>1</b> is connected to the second high voltage power supply <b>12</b> via the current adjustment circuit <b>40</b>. That is, the current adjustment circuit <b>40</b> is arranged between the second high-voltage power supply <b>12</b> and the reference circuit <b>20</b>.

p-0021The reference circuit <b>20</b> clamps the second power supply voltage VG supplied from the second high-voltage power supply <b>12</b> and generates reference voltage VR, which is higher than the output voltage OUT by an amount corresponding to the breakdown voltage of the Zener diodes D<b>1</b> and D<b>2</b>. That is, when the breakdown voltage of each of the Zener diodes D<b>1</b> and D<b>2</b> is represented by “VB”, the reference circuit <b>20</b> of the preferred embodiment generates a reference voltage VR of “OUT+2VB”. Accordingly, the reference voltage VR varies in a manner dependent on the output voltage OUT of the half-bridge circuit <b>100</b>.

p-0022The regulator circuit <b>30</b> is connected between the second high-voltage power supply <b>12</b> and the output terminal OP. The regulator circuit <b>30</b> includes an input terminal, which is connected to a node GN<b>1</b> (gate node) between the reference circuit <b>20</b> and the current adjustment circuit <b>40</b>, and an output terminal.

p-0023In the preferred embodiment, the regulator circuit <b>30</b> includes a push-pull circuit formed by two transistors <b>32</b> and <b>34</b>. The transistor <b>32</b> is formed by an NMOS transistor, and the transistor <b>34</b> is formed by a PMOS transistor. That is, the push-pull circuit has a CMOS push-pull configuration. The transistor <b>32</b> has a gate connected to the input terminal of the regulator circuit <b>30</b>, a source connected to the output terminal of the regulator circuit <b>30</b>, and a drain connected to the second high-voltage power supply <b>12</b> via a resistor R<b>2</b>. The transistor <b>34</b> has a gate connected to the input terminal of the regulator circuit <b>30</b>, a source connected to the output terminal of the regulator circuit <b>30</b>, and a drain connected to the anode of the Zener diode D<b>2</b>. Accordingly, the gates of the two transistors <b>32</b> and <b>34</b> are both supplied with reference voltage VR via the gate node GN<b>1</b>.

p-0024In response to the reference voltage VR, the push-pull circuit generates internal power supply voltage VPD, which is substantially higher than the output voltage OUT by a constant voltage. As mentioned above, the reference voltage VR varies in a manner dependent on the output voltage OUT and has a voltage level that is higher than the output voltage OUT. Thus, when an increase in the output voltage OUT increases the reference voltage VR, the transistor <b>32</b> is activated and the transistor <b>34</b> is deactivated. On the other hand, when a decrease in the output voltage OUT decreases the reference voltage VR, the transistor <b>32</b> is deactivated and the transistor <b>34</b> is activated. Accordingly, the transistors <b>32</b> and <b>34</b> operate in a complementary manner in accordance with the output voltage OUT, or the reference voltage VR.

p-0025When the transistor <b>32</b> is activated, if the threshold value of the transistor <b>32</b> is represented by “Vt”, the internal power supply voltage VPD of “VR−Vt” is output from a node between the transistors <b>32</b> and <b>34</b>, or the output terminal of the regulator circuit <b>30</b>. When the transistor <b>34</b> is activated, if the threshold value (absolute value) of the transistor <b>32</b> is also represented by “Vt”, internal power supply voltage VPD of “VR+Vt” is output from the output terminal of the regulator circuit <b>30</b>. As described above, the reference voltage VR is “VR=OUT+2VB”. Accordingly, the internal power supply voltage VPD varies within the range defined by the next expression. <br />OUT+(2<i>VB−Vt</i>)≦<i>VPD</i>≦OUT+(2<i>VB+Vt</i>)  Expression 1

p-0026As shown by expression 1, the push-pull circuit generates the internal power supply voltage VPD of OUT+(2VB±Vt) in response to the reference voltage VR. In the preferred embodiment, the reference voltage VR is set by the breakdown voltage 2VB of the Zener diodes D<b>1</b> and D<b>2</b> so that the maximum voltage “2VB+Vt”, which is added to the output voltage OUT by the push-pull circuit, does not exceed the gate voltage Vgst of the high-side transistor Q<b>1</b>. Here, Vt is smaller than 2VB. Accordingly, the internal power supply voltage VPD is substantially higher than the output voltage OUT by a constant voltage (gate withstand voltage Vgst).

p-0027The current adjustment circuit <b>40</b> includes the resistor R<b>1</b>. The resistor R<b>1</b> has one end connected to the second high-voltage power supply <b>12</b> and another end connected to the gate node GN<b>1</b>, that is, the input terminal of the push-pull circuit. The resistance value of the resistor R<b>1</b> determines the current Ir flowing from the second high-voltage power supply <b>12</b> to the gate node GN<b>1</b>. Accordingly, adjustment of the resistance value of the resistor R<b>1</b> changes the speed for charging a gate capacitor (coupling capacitor) of the transistors <b>32</b> and <b>34</b>. The resistance value of the resistor R<b>1</b> is increased to restrict the current Ir. This restricts the response speed of the push-pull circuit but reduces switching noise in the transistors <b>32</b> and <b>34</b>. Thus, the push-pull circuit generates internal power supply voltage VPD in a further stable manner. Further, the resistance value of the resistor R<b>1</b> is decreased to increase the current Ir flowing to the gate node GN<b>1</b>. This improves the response speed of the push-pull circuit. However, more switching noise may be produced. Accordingly, the resistance value of the resistor R<b>1</b> is set so as to trade off the response speed of the push-pull circuit with switching noise.

p-0028A capacitor C<b>1</b> is connected between the output terminal of the regulator circuit <b>30</b> (push-pull circuit) and the anode of the Zener diode D<b>2</b>. The capacitor C<b>1</b> stabilizes the internal power supply voltage VPD.

p-0029The level shifter <b>50</b> converts an input signal IN, which has a first amplitude level and oscillates between the voltage VDD and voltage VSS, to an output signal BIN, which has a second amplitude level and oscillates between the internal power supply voltage VPD and output voltage OUT. The input signal IN controls the activation and deactivation of the high-side transistor Q<b>1</b>.

p-0030The buffer circuit <b>60</b> is connected between the gate of the high-side transistor Q<b>1</b> and the level shifter <b>50</b>. Preferably, the buffer circuit <b>60</b> is connected to the gate of the transistor Q<b>1</b> via a resistor R<b>3</b>. Referring to <figref idrefs="DRAWINGS">FIG. 1</figref>, the buffer circuit <b>60</b> buffers the output signal BIN of the level shifter <b>50</b> based on the internal power supply voltage VPD and the output voltage OUT to stably generate the first drive voltage VDH. The first drive voltage VDH is supplied to the gate of the transistor Q<b>1</b>.

p-0031<figref idrefs="DRAWINGS">FIG. 2A</figref> is a waveform diagram showing the output voltage of the half-bridge circuit <b>100</b> (simulation values) and internal power supply voltage VPD (simulation values) of the pre-driver <b>10</b>. As shown in <figref idrefs="DRAWINGS">FIG. 2A</figref>, the output voltage varies in a manner dependent on the output voltage OUT. Further, as shown in <figref idrefs="DRAWINGS">FIG. 2B</figref>, the internal power supply voltage VPD has a level that is substantially higher than the output voltage by an amount corresponding to a constant voltage (corresponding to gate withstand voltage Vgst).

p-0032The pre-driver <b>10</b> of the preferred embodiment has the advantages described below.

p-0033In accordance with the output voltage OUT of the half-bridge circuit <b>100</b>, the push-pull circuit (<b>30</b>) generates the internal power supply voltage VPD that is substantially higher than the output voltage OUT by the gate withstand voltage Vgst. The buffer circuit <b>60</b> generates the first drive voltage VDH based on the internal power supply voltage VPD, which is generated by the push-pull circuit, and the output voltage OUT. This prevents the transistor Q<b>1</b> from being supplied with voltage exceeding the gate withstand voltage Vgst.

p-0034The pre-driver <b>10</b> uses the buffer circuit <b>60</b> to generate the first drive voltage VDH. This increases the responsiveness of the pre-driver <b>10</b> from that of a prior art pre-driver, which generates the drive voltage of the high-side transistor through constant current control.

p-0035The response speed of the push-pull circuit (<b>30</b>) may be changed by the current adjustment circuit <b>40</b> (resistor R<b>1</b>). In such a case, the resistance value of the resistor R<b>1</b> is lowered to increase the response speed. Thus, the circuit scale of the pre-driver is not enlarged. Further, even when increasing the response speed, the internal power supply voltage VPD does not become greater than or equal to “output voltage OUT+gate withstand voltage Vgst”. This ensures prevention of gate damaging.

p-0036It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms.

p-0037The capacitor C<b>1</b> may be eliminated.

p-0038The resistor R<b>2</b> may be eliminated.

p-0039The reference circuit <b>20</b> may be formed by a single Zener diode. Alternatively, the reference circuit <b>20</b> may be formed by three or more Zener diodes.

p-0040The size of the buffer circuit <b>60</b> (transistor size) and the size of the resistor R<b>3</b> (resistance value) may be varied to change the response speed of the pre-driver <b>10</b>.

p-0041The bridge driver is not limited to the half-bridge circuit <b>100</b>. The pre-driver <b>10</b> of the above embodiment may be used to drive the high-side transistor of a full-bridge circuit.

p-0042A diode-connected transistor may be used in lieu of Zener diodes in the reference circuit <b>20</b>. In this case, the reference circuit <b>20</b> includes at least one diode-connected transistor.

p-0043The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.